RTX support for Cycles
EDIT: It's possible now!

You have to get a newest 2.79 (aka Official) build from Blender Builder. It has CUDA support but the kernels are missing. That's easy to fix, however. Get a CUDA 10-based Linux build from GraphicAll. Then go to blender\2.79\scripts\addons\cycles\lib. Copy the filter_7*.cubin and kernel_sm_7*.cubin files to the corresponding folder in the Windows build. It should work now :)

Thanks to Carlo Andreacchio for showing this method in a comment on GraphicAll.



Original answer:

I tried to recompile Blender with CUDA 10.0. Sadly, the library that Blender uses for initializing CUDA (cuew) didn't even recognize the toolkit. Last commit is from 3 months ago, when CUDA 9.2 arrived. We have to wait...